/Engine/Source/Runtime/Slate/Public/Widgets/Layout/SScrollBar.h
Set the offset and size of the track's thumb. Note that the maximum offset is 1.0-ThumbSizeFraction. If the user can view 1/3 of the items in a single page, the maximum offset will be ~0.667f
virtual void SetState ( float InOffsetFraction, float InThumbSizeFraction, bool bCallOnUserScrolled )